There's some pretty useless stuff on the "at a glance" cards (ie, heat acclimation). I'm rather surprised "Recovery" (a metric that matters to pretty anyone and everyone) isn't a card? I only ever see it on my watch after a workout. If it's in the app, it's buried. I'd like to see that up front and available as knowing recovery time is 10x valuable than heat acclimation (which doesn't even work BTW).
it's an ultimately useless metric.
I cannot disagree that it is useless to you. However, I use it with my epix (Gen 2) and think it is a valuable tool. I'm sure I'm not the only one. Normally I'd say report it to Garmin Support, but if you do not care, it's all good. 
From what I can tell, heat acclimation is not an available feature on a Forerunner 255. Maybe it was added later?
Weird thing is that you even get that message that you need to train at 72° or hotter.
